In the mining industry, it is necessary to release potentially valuable minerals from their rock gangue for the purpose of concentrating them and extracting them.
In order to achieve this release, the ore must be crushed and finely ground.
In the grinding step alone, it may be estimated that 750,000 to 1 million tonnes of grinding media, in the form of spherical balls or cylpebs (frustoconical or cylindrical pebbles), are consumed annually in the world.
In grinding media, the following materials are mainly encountered:
1) low-alloy martensitic steels (0.7 to 1% carbon and alloy elements less than 1%) shaped by rolling or forging and then heat-treated in order to obtain a surface hardness of 60-65 RC; PA0 2) chromium-alloy martensitic cast iron (1.7 to 3.5% carbon and 9 to 30% chromium) shaped by casting and heat-treated in order to obtain a hardness of 60 to 68 RC throughout the cross-section; PA0 3) low-alloy pearlitic white cast irons (3 to 4.2% carbon and alloy elements less than 2%) not treated and having a hardness of 45 to 55 RC, obtained by casting.
Each of the current solutions has drawbacks which are specific to each of them:
for forged martensitic steels, the capital costs for forging or rolling machines, the heat-treatment plants and the energy consumptions are high; PA1 as regards chromium-alloy cast irons, there are additional costs related to the alloying elements (mainly chromium) and to the heat treatments; PA1 finally, for low-alloy pearlitic white cast irons, the manufacturing costs are generally quite low but the performance characteristics in terms of wear resistance are markedly inferior to the previous solutions. In addition, only grinding media of a size less than 60 mm are generally produced industrially. PA1 a non-equilibrium structure of fine pearlite, containing between 1 and 1.5% by weight of carbon with a hardness lying between 47 and 54 RC; PA1 a high carbon austenitic structure with a hardness lying between 15 and 30 RC; PA1 a high carbon martensitic structure with a hardness lying between 60 and 65 RC. PA1 between 1.3 and 1.7% as regards the steels consisting of fine pearlite; PA1 between 1 and 1.6% as regards the steels consisting of austenite; PA1 between 0.6 and 1% as regards the steels consisting of martensite.
More particularly, in the case of ores where the gangues are highly abrasive (for example: gold ore, copper ore, etc.), the current solutions are not entirely satisfactory for the users, since the contribution of the products and materials subjected to wear (balls and linings) remains large in the production costs of these potentially valuable metals.